Light petroleum fractions from refinery and petrochemical operations constitute a potentially valuable raw material source for the production of styrene and other commercially important aromatic compounds.
Processes for the conversion of light paraffins to aromatic hydrocarbons at some stage usually involve naphthenic compounds. A well-known reaction is the dehydrocyclization of C.sub.6 and higher paraffins to aromatic derivatives containing the same number or less carbon atoms than the feed material.
More recent development efforts have been concerned with a dehydrocyclodimerization mechanism of reaction, wherein a light olefinic hydrocarbon such as butadiene is dimerized to vinyl cyclohexene, which intermediate is subsequently sujected to catalytic dehydrogenation conditions to yield product mixtures of aromatic compounds such as benzenes, ethylbenzene, xylene, styrene, methylstyrene, naphthalene, and the like.
Efforts to develop economical processes for selective production of styrene from readily available hydrocarbon raw materials have encountered many difficulties. Several prior art methods have been advanced which address the most serious of the technical problems.
U.S. Pat. No. 2,376,985 describes a two-step process for converting butadiene to styrene. In the first step, butadiene is heated at 250.degree.-500.degree. C. under 1-10 atomspheres pressure preferably in the presence of a catalyst such as chromium oxide or silica gel to form butadiene dimer. In the second step, the butadiene dimer is automized in the presence of a Group V, VI or VIII metal catalyst. The second step is conducted at a temperature of 450.degree.-800.degree. C. and pressure of 0.2-2 atmospheres for a reaction period of 0.1-100 seconds.
U.S. Pat. No. 2,392,960 describes a cyclic process for the production of styrene which involves (1) subjecting a butane-butene-butadiene feed to a temperature of 650.degree.-950.degree. C. under superatmospheric pressure to effect dimerization of the butadiene component, and (2) contacting the resultant effluent from the first step with chromium or molybdenum metal catalyst at a temperature of 1200.degree.-1400.degree. F. and a pressure of 1 atmosphere for a 0.001-0.1 second reaction time to yield styrene. A proportion of the butane and butene components of the reaction mixture are converted to butadiene during the dehydrogenation step and recycled to the first step of the process.
U.S. Pat. No. 2,438,041 describes a process for producing styrene which comprises (1) contacting butadiene at 150.degree.-480.degree. C. and 20-30 atmospheres with a catalyst selected from Fuller's earth, bauxite, alumina and silica gel to form an effluent which contains butadiene dimer, (2) separating tar from the effluent, and (3) dehydrogenating the butadiene dimer component of the effluent over a chromium oxide on alumina catalyst at a temperature of 400.degree.-600.degree. C. and a pressure of at least 1 atmosphere. In the illustrated Example, the resultant product mixture contained 13.8 weight percent of styrene and 5.3 percent ethylbenzene.
U.S. Pat. No. 3,502,736 describes on improved method for oxidative dehydrogenation of alkenylnaphthenes to alkenylaryl compounds. A non-aromatic cyclic hydrocarbon having at least one unsaturated bond in a sidechain (e.g., vinylcyclohexene) is contacted with a palladium oxyhydrate catalyst in the presence of oxygen to yield the corresponding alkenylaryl derivative (e.g., styrene).
S.M. Csicsery reports extensive dehydrocyclodimerization studies in a I-V series of articles in the Journal of Catalysis [17, 207, 216, 315, 323; 18, 30 (1970)]. C.sub.3 -C.sub.5 paraffins were converted in one step to aromatics at temperatures over 430.degree. C. in the presence of dual-functioning catalyst having dehydrogenation and acid-type activities (e.g, platinum on acidic alumina). The products from butanes were principally xylene and toluene. Ethylbenzene and styrene were the predominant C.sub.8 aromatics formed from n-butane over weakly acidic catalysts. Dehydrocyclodimerization of propane over supported platinum catalysts produced benzene as the principle aromatic product, and pentane yielded naphthalenes and other condensed aromatics. At very short contact times over Pt-alumina catalysts butenes converted to C.sub.5 -C.sub.8 olefins and naphthenes in large proportion, which suggested that these compounds were intermediates in butene aromatization. In Paper V it is theorized that dehydrocyclodimerization proceeds by (1) conversion of the light paraffins to monoolefins and diolefins, (2) dimerization of the olefins, (3) aromatization of the dimerized olefins, and (4) the isomerization, transalkylation and dealkylation of the primary aromatics to produce a large number of C.sub.8 -C.sub.10 alkylbenzene isomers.
There remains a need for improved methods for converting low value light hydrocarbon overhead streams from petroleum refinery operations into more valuable derivatives such as naphthenes and aromatic hydrocarbons.
Accordingly, it is an object of this invention to provide an improved process for converting light acyclic hydrocarbons into cyclic hydrocarbons.
It is another object of this invention to provide an improved process for converting n-butenes into naphthenes and aromatic hydrocarbons.
It is a further object of this invention to provide an improved two-step process for converting n-butenes into styrene.
Other objects and advantages shall become apparent from the following description and examples.